As the day goes on, we'll find out what players, if any, are being moved. July 31st has only been a landmark day since really 1989; prior to that, many deals were completed before that. The trading deadline was started by Kenesaw Landis after a late season 1922 trade brought Joe Dugan to the Yankees which allowed them to win the AL Pennant. The deadline was originally June 15th and it stayed that way from 1923-1985. It became July 31st for the 1986 season.
The first major July 31st trade was when the New York Mets acquired LHP Frank Viola from the Minnesota Twins for five players, including RHP Rick Aguilera and Kevin Tapani. While the Mets were making a push to return to the postseason, it was the Twins who reaped the benefits of this trade, winning the World Series just two years later.
Just five years ago, one of the more significant trades was made with time winding down. The Atlanta Braves added 1B Mark Teixeira from the Texas Rangers, giving up SS Elvis Andrus, LHP Matt Harrison, RHP Neftali Feliz and C Jarrod Saltalamacchia in a 5 for 1 trade. The Rangers owe part of the credit for their consecutive World Series appearances to that trade. In 1997, the Boston Red Sox acquired RHP Heathcliff Slocomb from the Mariners for C Jason Varitek and RHP Derrick Lowe. It is possible the Red Sox do not have their success for the next 10 + seasons without that trade, among other things.
While the last trade gave the Red Sox two important pieces for their championship runs, it was the 2004 trade of star SS Nomar Garciaparra to the Cubs; the four team deal that brought the Red Sox SS Orlando Cabrera and 1B Doug Mientkiewicz, that may have been the turning point of the Red Sox fortune.
Several sluggers have been moved on July 31st. The Yankees acquired Cecil Fielder from the Tigers in 1996, and Greg Vaughn was moved from the Brewers to the Padres on the same day. Mark McGwire was moved from Oakland to St Louis exactly one year later. 2008 saw Ken Griffey, Jr move from the Reds to the White Sox and Manny Ramirez move from the Red Sox to the Dodgers. And for good measure, Seattle traded future HOF LHP Randy Johnson to the Astros for INF Carlos Guillen, RHP Freddy Garcia and LHP John Halama on July 31st of 1998.
Another interesting trade was made in 1997, when the Chicago White Sox threw up the white flag when they traded LHP Wilson Alvarez, RHP Roberto Hernandez and RHP Danny Darwin to the San Francisco Giants for six players, including Bob Howry and Keith Foulke. This resulted in 3B and current White Sox manager Robin Ventura to say, "I didn't know the season ended August 1st!"
Many teams have made minor tweaks on the last day before players have to clear waivers before they can be traded. This works until August 31, afterwards no player added can be part of a team's postseason roster. As the time runs down today, it will be interesting to see what deals, if any will be made today.